Piano-style keyboards employed in electronic musical instruments have operating environments different from those of acoustical instruments. Conventional piano keys are designed to deliver hammer blows, while conventional organ and accordion keys are designed to open air valves. In contrast, the keys of electronic organs, synthesizers, etc. are designed simply to operate electrical switches which impose little or no mechanical load on the key action. Therefore electronic musical instrument keys typically require return spring mechanisms strong enough to provide some tactile sensation of resistance when the keys are depressed, as well as to return the keys to their original positions after they are released.
In the past, such return spring mechanisms have suffered from excessive "build-up"; i.e. the force exerted by the return spring in opposition to the musician's fingers increases significantly as the key gets further into its downstroke. It has not been unusual for some prior art key return spring mechanisms to exert twice as large an opposing force at the end of the key stroke as they do at the beginning. This degree of force build-up makes it more difficult to play the instrument.
One way to reduce force build-up is to locate the spring biasing point closer to the fulcrum of the key, thus reducing the moment arm over which the spring acts, and also reducing the amount of spring deflection. Since the torque exerted on the key is proportional to both the moment arm and the spring deflection, both factors affect the force build-up.
In the past some return spring mechanisms, such as the one shown in U.S. Pat. No. 4,128,035, have employed coil springs located at the rear of the key, far from the fulcrum. Such coil springs can be relocated closer to the fulcrum, but that tends to make them inaccessible. However, coil springs are quite inexpensive, and it is desirable that any alternative mechanism remain competitive in cost.
Locating the return spring at the rear of the key can be disadvantageous in some other respects. Recently there have been certain advances in the design of key switches. These advances can best be realized by locating each such switch at the rear of the key which actuates it. The best way to make room at the rear of the key for the installation of such a switch mechanism is by removing the return spring from its rear location, and relocating it to some other part of the key.
Another problem with prior art key return spring mechanisms is that they are often difficult to assemble, or to reassemble when replacement of the return spring proves necessary. In particular, prior art mechanisms usually locate the return spring somewhere below and behind the keyboard, which makes access difficult. It also obscures the fact that a spring has broken or become dislodged from its operating position.